(1) Field of the Invention
The present invention relates to a semiconductor chip and a semiconductor device that includes a mounting circuit board on which the semiconductor chip is flip-chip mounted, and in particular to a high frequency semiconductor integrated circuit such as a monolithic microwave integrated circuit (MMIC) and the like which includes a transmission line.
(2) Description of the Related Art
With the development of communication devices including mobile telephones, a circuit device with which a microwave having a frequency ranging from hundreds of MHz to several GHz and a millimeter wave having a frequency ranging from dozens of GHz to over 100 GHz are handled, has been attracting attention. With the demand for a wide frequency band which allows communication of larger amount or for performing signal processing at higher speed, a desired operating frequency of a circuit is getting higher and higher.
When a frequency becomes higher, a wavelength of a signal becomes close to an actual size of a circuit element. Then, it becomes difficult to handle the circuit element as a lumped parameter element. This necessitates incorporating the size of the circuit into design as a distributed parameter element. In addition, since ununiformity in the shape or mount of the circuit device leads directly to unevenness of frequency characteristics, mounting many components makes it difficult to hold the characteristics of the entire circuit device within standards.
In view of the above circumstances, the technique of a monolithic microwave integrated circuit (MMIC) is used for such a high frequency circuit. With the technique, a passive element such as a transmission line is collectively manufactured on the same semiconductor substrate together with a transistor that is an active element. The reason for this is that it is possible to reduce the number of components by collectively manufacturing, on the substrate, lots of circuit components including the passive element, and to accurately reproduce the characteristics even when the wavelength becomes small, by employing a semiconductor process which enables accurate microfabrication.
A compound semiconductor including GaAs has mainly been used so far, as a semiconductor for MMIC as described above. The primal reason is that it is possible to obtain excellent high frequency characteristics of a transistor resulting from high electron mobility and a low-loss substrate with high insulation properties. However, the substrate of such a compound semiconductor is expensive compared to a generally used Si semiconductor (semiconductor including Si as a main component), and the manufacturing process is still undeveloped in many points compared to a silicon process preceding in mass production. Thus, there is a problem of higher costs in terms of yield ratio as well.
With the recent development in miniaturization techniques, however, the operating frequency of the Si semiconductor has significantly improved. It is reported that the maximum cutoff frequency (ft) and the maximum oscillating frequency (fmax) of a leading-edge microscopic metal-oxide semiconductor (MOS) transistor or a SiGe hetero bipolar transistor has exceeded 100 GHz and has reached 200 GHz. This has led active research and developments to be carried out in various places, with which an MMIC of a microwave band to a millimeter wave band which has been manufactured using an expensive compound semiconductor is to be replaced with a low-cost Si semiconductor.
However, there is a problem in the Si semiconductor in that it is difficult to manufacture a substrate having excellent insulation properties as in a compound semiconductor such as GaAs or the like. More specifically, with the compound semiconductor such as GaAs or the like, for example, it is possible to constitute a microstrip line, as illustrated in FIG. 8, by forming a line 91 on a substrate 90 with the semi-insulating substrate 90 itself being a dielectric body and forming a GND plane (ground conductor) 92 whose bottom surface is metallized, thereby manufacturing a low-loss circuit. However, when the same configuration is applied to the Si substrate, since the Si substrate is conductive in general, a loss in the line significantly increases when an electromagnetic field occurring in the line formed on the Si substrate invades in the substrate.
In order to solve the above-described problem, a multilayer interconnection technique of a silicon integrated circuit is conventionally used for efforts to prevent the electromagnetic field occurring in the line from invading in the Si substrate 93, by constituting a microstrip line including a first layer wire 96 formed on the Si substrate 93 as a ground conductor (GND plane) and an uppermost layer wire 94 as a signal line as illustrated in FIG. 9, for example. This allows the loss of the dielectric body due to conductivity of the Si substrate 93 to be eliminated. However, since an interlayer insulating film 95 used as the dielectric body is substantially thin compared to the semiconductor substrate 93 according to this method, it is necessary to reduce the line width of the signal line in order to obtain the same characteristic impedance as that in the case where the semiconductor substrate 93 is used as the dielectric body. However, since a conductor loss of the signal line increases when the line width is reduced, the loss as the transmission line becomes larger, after all, compared to the compound semiconductor.
Here, it is effective to broaden the line width of the signal line for reducing the conductor loss. However, in order to realize this without decreasing the impedance, it is necessary to thicken the interlayer insulating film that is the dielectric body. As a conventional example of the above-described technique, a technique is proposed by, for example, Patent Literature 1 (Japanese Unexamined Patent Application Publication No. 9-17959), with which a dielectric film such as benzocyclobutene (BCB) or the like which is different from a general interlayer insulating film is deposited for several μm to dozens of μm to constitute the microstrip line using the dielectric film as a dielectric body. This technique is directed to eliminate the loss of the dielectric body by thickening the dielectric film to broaden the line width of the signal line which realizes the same impedance so as to reduce the conductor loss, and by further including a GND plane on the Si substrate to prevent the electromagnetic field from invading in the Si substrate. This implements an MMIC (hereinafter also referred to as “Si-MMIC”) which includes a Si semiconductor as a substrate.
However, even with the improvement disclosed by Patent Literature 1, a new problem occurs when a circuit device is to be configured by mounting, in practice, the Si-MMIC on a circuit board.
FIG. 10 is a diagram which schematically shows a sectional structure of a conventional Si-MMIC disclosed by Patent Literature 1 and the like, which is mounted on a circuit board 98 which is used for mounting. A signal line on the MMIC and a signal line on the circuit board are usually connected using a metal wire having a diameter of approximately 25 μm. In addition, since the GND plane 96 on the MMIC is usually insulated from the Si substrate 93, it is necessary to be connected to a GND pattern on the circuit substrate 98 through wire bonding, in the same manner as in the signal line.
Here, as clearly shown by the diagram, since there is a wire 97 with a finite length for the connection between the GND on the MMIC and the GND on the circuit board 98, inductance of the wire 97 cannot be ignored especially in high frequency such as a millimeter wave. The GND plane 96 on the MMIC is in a state floating from the GND via the inductance, and the GND which is supposed to supply a stable potential significantly fluctuates according to a signal, in some cases. This forms a feedback loop in the entire mounting circuit and in some cases causes unnecessary oscillation.
It is to be noted that, since the substrate itself is used as a dielectric body in a conventional MMIC in which a compound semiconductor such as GaAs or the like is used, the GND plane is located on a lower surface of the substrate, and thus it is possible to obtain a stable connection with the circuit substrate. Furthermore, the GND wire on the circuit is also connected to the GND on the bottom surface, usually through via hole, and thus it is possible to minimize a parasitic inductance. With the Si-MMIC as in the conventional technique described above, since the GND is located on the upper surface of the substrate, a large parasitic inductance inevitably occurs in the connection between the GND of the MMIC and the GND of the circuit substrate.
As described above, with the Si-MMIC, the parasitic inductance inevitably occurs in the connection between the GND plane on the upper surface of the MMIC substrate and the circuit substrate, and thus the GND on the MMIC becomes unstable, causing the characteristics to change. Furthermore, there is a problem that it is likely to cause oscillation or the like.